Ion implantation is a standard technique for introducing conductivity-altering impurities into a workpiece. A desired impurity material is ionized in an ion source, the ions are accelerated to form an ion beam of prescribed energy, and the ion beam is directed at the surface of the workpiece. The energetic ions in the beam penetrate into the bulk of the workpiece material and are embedded into the crystalline lattice of the workpiece material to form a region of desired conductivity.
Solar cells are one example of a device that uses silicon workpieces. Any reduced cost to the manufacture or production of high-performance solar cells or any efficiency improvement to high-performance solar cells would have a positive impact on the implementation of solar cells worldwide. This will enable the wider availability of this clean energy technology.
Solar cells are typically manufactured using the same processes used for other semiconductor devices, often using silicon as the substrate material. A semiconductor solar cell is a simple device having an in-built electric field that separates the charge carriers generated through the absorption of photons in the semiconductor material. This electric field is typically created through the formation of a p-n junction (diode) which is created by differential doping of the semiconductor material. Doping a part of the semiconductor substrate (e.g. surface region) with impurities of opposite polarity forms a p-n junction that may be used as a photovoltaic device converting light into electricity.
FIG. 1 shows a cross section of a representative solar cell 100, where the p-n junction 120 is located away from the illuminated surface. Photons 10 enter the solar cell 100 through the top (or illuminated) surface, as signified by the arrows. These photons pass through an anti-reflective coating 104, designed to maximize the number of photons that penetrate the substrate 100 and minimize those that are reflected away from the substrate. The ARC 104 may be comprised of an SiNx layer. Beneath the ARC 104 may be a passivation layer 103, which may be composed of silicon dioxide. Of course, other dielectrics may be used. On the back side of the solar cell 100 are an aluminum emitter region 106 and an aluminum layer 107. Such a design may be referred to as an Al back emitter cell in one instance.
Internally, the solar cell 100 is formed so as to have a p-n junction 120. This junction is shown as being substantially parallel to the bottom surface of the solar cell 100, although there are other implementations where the junction may not be parallel to the surface. In some embodiments, the solar cell 100 is fabricated using an n-type substrate 101. The photons 10 enter the solar cell 100 through the n+ doped region, also known as the front surface field (FSF) 102. The photons with sufficient energy (above the bandgap of the semiconductor) are able to promote an electron within the semiconductor material's valence band to the conduction band. Associated with this free electron is a corresponding positively charged hole in the valence band. In order to generate a photocurrent that can drive an external load, these electron-hole (e-h) pairs need to be separated. This is done through the built-in electric field at the p-n junction 120. Thus, any e-h pairs that are generated in the depletion region of the p-n junction 120 get separated, as are any other minority carriers that diffuse to the depletion region of the device. Since a majority of the incident photons 10 are absorbed in near surface regions of the solar cell 100, the minority carriers generated in the emitter need to diffuse to the depletion region and get swept across to the other side.
Some photons 10 pass through the front surface field 102 and enter the p-type emitter 106. These photons 10 can then excite electrons within the p-type emitter 106, which are free to move into the front surface field 102. The associated holes remain in the emitter 106. As a result of the charge separation caused by the presence of this p-n junction 120, the extra carriers (electrons and holes) generated by the photons 10 can then be used to drive an external load to complete the circuit.
By externally connecting the base through the front surface field 102 to the emitter 106 through an external load, it is possible to conduct current and therefore provide power. To achieve this, contacts 105, typically metallic and in some embodiments silver, are placed on the outer surface of the front surface field 102.
It may be advantageous to more heavily dope the regions on which the contacts 105 interface. These regions may be made by using implantation in conjunction with a traditional lithographic mask, which can then be removed easily before dopant activation. Another alternative is to use a shadow mask in the implanter to define the highly doped areas for the contacts. All of these techniques utilize a fixed masking layer (either directly on the substrate or in the beamline). In addition to traditional beamline or plasma deposition systems, other implant systems may be used. For example, a flood ion implanter without mass analysis or a plasma tool that focuses ions by modifying the plasma sheath may also be used.
While blanket doping steps may use an ion implanter or other processing steps, selective implantation may use a mask or an ion beam modified by the plasma sheath. Turning to FIG. 2, a cross-sectional diagram of selective implantation is illustrated. When a specific pattern of ion implantation or doping in a workpiece 200, such as a solar cell, is desired, then a mask 201 may be used. This mask 201 may be a shadow or proximity mask. The mask 201 is placed in front of a substrate 200 in the path of a species 204 during implantation. This species 204 may be a dopant. The substrate 200 may be placed on a platen 203, which may use electrostatic or physical force to retain the substrate 200. The mask 201 has apertures 202 that correspond to the desired pattern of ion implantation in the substrate 200. The apertures 202 may be stripes, dots, or other shapes. While the mask 201 is illustrated, photoresist, other hard masks, or other methods including but not limited to using ion beams modified by the plasma sheath known to those skilled in the art likewise may be used in an alternate embodiment.
An enhancement to solar cells is the addition of heavily doped substrate contact regions. Turning back to FIG. 1, these heavily doped contact regions correspond to the areas where the metallic fingers 105 will be affixed to the solar cell 100. The introduction of these heavily doped contact regions allows much better electrical contact between the solar cell 100 and the metallic contacts 105 and significantly lowers the series resistance of the solar cell 100. The use of heavily doped regions on the surface of the emitter are referred to as a selective emitter design. Similarly, the use of heavily doped contact regions on a surface field is referred to as selective front surface field (FSF) or back surface field (BSF) design.
A selective emitter, FSF, or BSF design for a solar cell also has the advantage of higher efficiency cells due to reduced minority carrier losses through recombination due to lower dopant/impurity dose in the exposed regions of the emitter layer. The higher doping under the contact regions provides a field that repels the minority carriers generated in the emitter or base and pushes them towards the p-n junction thus reducing the recombination losses of minority carriers to the metal contacts.
A second enhancement to solar cells is the creation of bifacial solar cells. Bifacial solar cells are configured to produce power by absorbing light from two sides of the solar cell. Thus, instead on having a solid layer of aluminum (or other material) on the back surface, as shown in FIG. 1, a grid or pattern of contacts may be used.
Previously, mirrors or reflectors were required to collect sunlight on more than one side of a solar cell because the solar cells were only able to accept photons introduced on one side. Use of more than one side of a solar cell increases solar cell efficiency. Another advantage of a bifacial solar cell is the fact that the power output is less sensitive to the time of the day since the light scattered at non-normal incidences gets back reflected toward the solar cell, thus improving the levelized cost of electricity (LCOE) equation of such solar cells. One example of a place where such modules would be preferred would be near sandy beaches or other sandy locations where the light reflected from the sands would be absorbed on the underside of a bifacial solar cell integrated into the ceiling of an overhead structure.
Therefore, there is a need in the art for an improved bifacial solar cell to maximize the energy that can be produced from incident photons.